Project Summary Neuropeptide S (NPS) and its receptor (NPSR) comprise a recently deorphanized G protein-coupled receptor system that has been implicated in stress and anxiety (1, 2). Moreover, there is gathering evidence for a role of this receptor in addiction. For example, the NPSR has been shown to affect ethanol consumption, morphine place preference, and cue induced relapse to cocaine. Humans express two common allelic variants of the NPSR, N107 and I107 (the primordial allele expressed in mice). NPS shows enhanced activity at variant I107 despite no change in agonist affinity leaving a mystery as to how this change is affected. We have found that the trafficking profiles of the N107 and I107 alleles of the NPSR variants differ. In particular, we have found that, not only is NPS more potent on the I107 variant, but that this receptor is recycled after endocytosis while the N107 variant is degraded. Consequently, we predict that the anxiolytic properties of NPS will be enhanced when the I107 allele is expressed. By extension, if the anxiolytic properties of the endogenous NPS/NPSR system modulate drug use/abuse or relapse, one would expect the N107 allele to increase risk of drug seeking and/or relapse. Because N107 is a common allelic variant (expressed in ~40% of humans) it is therefore important to determine whether this locus is risk factor for drug abuse. While condition place preference (CPP) and drug self-administration are often used as paradigms for modeling ?addiction? in rodents, it is clear from several studies in rat that the transition to compulsive drug seeking can be dissociated from both reward and consumption, suggesting that neither of these models are good surrogates for ?addiction?. Because of our interests in the role of genetic variation in drug abuse liability, we have developed a novel model of the transition from controlled to compulsive opioid drug use in mice that recapitulates many features of human addiction, including persistent drug seeking despite adverse consequences and a decreased preference for alternative rewards. Here we propose to use this model together with knock-in mice expressing the N107 allele of the NPSR and their wild type littermates to examine whether variation at this locus alters risk for progression to addiction like behavior and/or relapse to drug seeking.